The disclosure relates to a pixel unit, and more particularly to a pixel unit having thin-film transistors.
FIG. 1 is a schematic diagram of a conventional pixel unit. A thin-film transistor (TFT) 101 comprises a gate receiving a scan signal S1, a drain receiving a data signal D1, and a source. A TFT 103 comprises a gate coupled to the source of TFT 101, a drain, and a source coupled to a voltage source Vdd. A capacitor 105 is coupled between the gate of TFT 103 and the source of TFT 103. A light-emitting element 107 is coupled between the drain of TFT 103 and a voltage source Gnd.
TFT 101 is turned on as scan signal S1 is asserted. Therefore, data signal D1 is input to capacitor 105 such that capacitor 105 is charged. When voltage stored by capacitor 105 reaches a preset value, TFT 103 is turned on such that light-emitting element 107 is lit.
If TFT 103 is desired to operate in a saturation region, voltage difference Vds across the drain of TFT 103 and the source thereof is defined in the following.Vds>Vgs−Vth; 
Wherein, Vgs is voltage difference across the gate of TFT 103 and the source of thereof and Vth is threshold voltage of TFT 103.
Assuming Vgs equals −5V and Vth equals −1.5V. To operate TFT 103 in the saturation region, the voltage difference Vds is required to exceed 3.5V.
If voltage difference across light emitting element 107 equals 6V, light-emitting element 107 displays maximum brightness. Therefore, voltage difference V103 between nodes N1 and N2 is required to exceed 9.5V such that TFT 103 can be operated in the saturation region and light-emitting element 107 displays maximum brightness.